Interesting about the 'size & reasonable terms' of the bond. Fortunately, I never had need to test it, but I was wildly p@#$%d off be interviewed for a UK airline in April; find that colleagues at the same interview had joined in June on an X bond, only for me to join in September on a 2X bond. Same training, same everything. A brick wall awaited me in the C.P & HR offices when I tried to discover why.
There was one company who's bond read "...you will be charged for your training costs etc etc." When numerous people left the bond was attempted to be invoked. A fixed price was quoted for everyone, but it was obvious that each pilot had incurred different costs due to different hours on the a/c for base training & line training etc. Thus each individual asked for an itemised bill for their bond, as had been stated in the bond; "your training costs." The company could not do so to the satisfaction of a judge and the whole matter was dropped. Perhaps the fact that these lucky guys were leaving a sinking charter ship and moving to the luxury liner of the local major meant they had time and support for the whole affair, and didn't give a rat's a@#e for references. They already had the job and the major enjoyed shaffting the charter opposition.
So, it would seem that unless the bond included a specific amount, which they could show to be less than your training costs, you might have a case. I wonder what the case would be if the bond was higher than your training costs? I know of one airline that has tried that!!